1. Field of the Invention
The present invention relates to an air inlet of an aeroengine, and in particular an aeroengine having unducted pusher propellers (also known as an “open-rotor pusher” or a “pusher unducted fan”).
2. Description of the Related Art
An engine of that type has two contrarotating turbines, each of which is constrained to rotate with a propeller that is situated outside the nacelle of the engine, the two propellers being arranged on a common axis one behind the other at the downstream end of the engine.
The engine is connected to an airplane via a pylon that extends substantially radially relative to the longitudinal axis of the engine and that has its radially inner end connected to the upstream end of the nacelle of the engine, i.e. to the air inlet of the engine.
The pylon must be at a sufficient axial distance from the propellers and from the upstream end or leading edge of the air inlet, in particular for aerodynamic reasons. In the prior art, it is necessary to lengthen the air inlet of the engine in the axial direction in order to enable the pylon to be connected to the air inlet and to the engine, and that gives rise to a significant increase in the weight of the nacelle and in the drag it generates in operation.
In the prior art, the air inlet of an engine having unducted pusher propellers is generally axisymmetric, i.e. its leading edge lies in a plane perpendicular to the axis of the engine. An axisymmetric air inlet has a ratio L/D that is constant all around its circumference, where L is the local length of the air inlet measured parallel to the axis of the engine between a point of the leading edge and a plane located at the upstream rotor of the engine, and where D is the inside diameter of the air inlet, at the position of said upstream rotor.
When the air inlet is not axisymmetric, its leading edge defines a surface that is approximately plane and that is referred to as a “capture” section (also known as a “hilite” or “high light”). That type of air inlet has a defined overall air inlet length that is equal to the distance between the transverse plane at the upstream rotor of the engine and the point of intersection between the capture plane of the air inlet and the axis of the engine.
For example, aeroengine air inlets are known that are of a chamfered (or “scarfed”) type in which the capture plane presents a marked angle of inclination relative to the longitudinal axis of the engine, the bottom portion of the air inlet projecting axially upstream relative to its top portion. An air inlet of that type is defined by a “global” L/D ratio (global length divided by diameter) and it presents a “local” L/D ratio (local length divided by diameter) that varies linearly around the circumference of the air inlet.
That particular chamfered shape for the air inlet serves essentially to limit the noise emitted upstream by the engine towards the ground. The longer bottom portion of the chamfered air inlet serves to reflect and deflect upwards a large fraction of the noise that is emitted upstream by the engine while it is in operation. That type of engine is generally connected under the wing of an airplane by means of a pylon that is connected to the air inlet via its shorter top portion.